It is well known that in the manufacture of rubber products crude or raw rubber is compounded with various ingredients, among which are sulfur and accelerators of vulcanization. The primary function of an accelerator or any combination thereof, is to increase the rate of the vulcanization process, consistent with sufficient time to mix the accelerators into the rubber at an elevated temperature before vulcanization commences, commonly referred to as scorch time. Secondary effects are almost of equal importance and include improved tensile strength, set, hysteresis and aging properties, among others. However, from the rubber compounders point of view, behavior is more important than chemical composition, and he is more likely to be concerned with cure rate and cure time, scorch behavior; the extent of cure, and compatibility, when using any one or combination of accelerators in a vulcanization process.
An accelerator may provide too slow a cure rate when used alone. It may be speeded up by adding small amounts of another accelerator. This added accelerator is called an "activator" and its use is called "activation." The accelerators which are most commonly "activated" are the thiazoles, including thiazole sulfenamides. The accelerators most commonly used as "activators" are the thiuram sulfides, dithiocarbamates and guanidines. The common ratios of about one part per hundred of rubber of the thiazole to 0.1 to about 0.3 part per hundred of rubber of the "activator" leads to the characterization of the thiazoles as "primary accelerators" and to the activators as "secondary accelerators."
Among the "primary accelerators" benzothiazole sulfenamides are commonly used in the vulcanization of a wide range of diene rubber compositions. They are frequently used in combination with thiuram sulfide and dithiocarbamate "secondary accelerators," which are powerful activators and tend to be fast curing and "scorchy," i.e., they tend to cause premature vulcanization. This is evidenced by the small amounts which are required for activation. A ratio of 10 to 1 of thiazole sulfenamide to activator is common practice.
One of the most widely used accelerator combinations for diene rubbers consists of a benzothiazole sulfenamide and a thiuram sulfide or dithiocarbamate. It is safe, that is not scorchy, provides adequate rate of cure in a reasonable length of time, and results in rubber compositions with good physical properties. Nevertheless, in commercial operations, e.g., in the manufacture of automobile tires, it is always desirable to improve productivity and cost. One means for increasing productivity is to reduce vulcanization time without sacrifice in quality or operating safety. In the past, various cure systems have been evaluated, some of which significantly increased the rate of cure, but at a cost in reduced processing safety, scorch, which has restricted their use.